A process for detecting contaminants in yarn is disclosed in EP-A-0,197,763, and its counterpart U.S. Pat. No. 4,739,176. In this process, a background surrounds the test material in the manner of a guide slot and is subjected to light. The background is coordinated with the test material in such a way that the total quantity of light reflected from the test material and of light coming from the background is independent of the dimensions and density of the test material and on the distribution of the fibers within the latter. It is intended to be possible in this way for a variation in the reflected light to indicate a foreign substance and not a variation in the dimensions, the density or the fiber distribution in the test material.
In this process, whenever the kind or type of test material is changed, relatively complex adjustment work is necessary to adapt the background to the test material. Also, the process is highly sensitive to contamination and aging of the background. Moreover, both of these are phenomena which are not only unavoidable in a textile concern but frequently occur in that very environment.